1. Field of the Invention
The present invention relates generally to the production, separation and/or recovery of lactic acid and more particularly to the production, separation and recovery of lactic acid via a fermentation process and the separation and/or recovery of lactic acid from a lactate feed solution such as is obtained from a fermentation broth or other sources.
2. Description of the Prior Art
Lactic acid has long been used as a food additive and in various chemical and pharmaceutical applications. More recently, lactic acid has been used in the making of biodegradable polymers both as a replacement for present plastic materials as well as various new uses where biodegradability is needed or desired. Accordingly, there is an ever increasing demand for lactic acid. The present invention aims at meeting this demand by providing an efficient and environmentally friendly process for producing lactic acid, that can, if desired, be employed to avoid the consumption of bases and acids; and, that can be used, if desired, to substantially reduce, if not eliminate, the formation of waste or byproduct salts.
Production of lactic acid is commonly carried out by fermentation of a strain of the bacterial genus Lactobacillus and more particularly by the species Lactobacillus delbrueckii or Lactobacillus acidophilus as examples. In general, the production of lactic acid by fermentation in a fermentation broth is well known. The fermentation substrate consists of carbohydrates together with suitable mineral and proteinaceous nutrients. Because the lactic acid producing microorganisms are inhibited in a strongly acidic environment, the pH of the fermentation broth must be kept above 4.5, and preferably within the range of about 5.0 to 7.0, more preferably within the range of about 5.5 to 6.5, and most preferably within the range of about 6.0 to 6.5. To maintain this pH level, suitable water-soluble basic substances or agents that are non-toxic to the acid producing microorganism, such as alkali metal hydroxides, carbonates or bicarbonates or alkaline earth metal hydroxides or carbonates, are commonly added to the fermentation broth to neutralize the acid being produced. This results in the formation of a lactate solution rather than the desired lactic acid product. Such lactate solution contains the lactate anion and the corresponding cation of the substance used to neutralize the fermentation broth.
Various methods have been proposed for the recovery of lactic acid from a fermentation broth. Where the fermentation is carried out in the presence of calcium carbonate, it is possible to recover the lactic acid by acidification with sulfuric acid. This results in the precipitation of calcium sulfate, while free lactic acid remains in the mother liquor. If desired, the mother liquor may be concentrated to up to about 90% by weight lactic acid. Subsequently, lactic acid may be extracted from the mother liquor with a suitable organic extractant, to yield an extract which is later back-extracted with water, or the acid may be adsorbed on a suitable adsorbent and later desorbed. The resulting aqueous lactic acid solution may then be concentrated. This method has the disadvantage, however, that it irreversibly consumes calcium carbonate and sulfuric acid and leaves, as waste, large quantities of calcium sulfate, which can give rise to disposal problems.
U.S. Pat. No. 5,132,456 (King et al.) describes a process for recovering carboxylic acid from a carboxylic acid-containing aqueous feed stream having a pH close to or above the pKa level of the acid. In accordance with that process the recovery involves what may be described as a cascade type acid withdrawal operation in which the basicity of the extractant is increased stepwise. In a first stage of the process, the feed stream is contacted with an adsorbent such as a strongly basic extractant or a solid anion exchanger. In a second stage the acid-loaded adsorbent is contacted with an aqueous solution of ammonia or a low molecular weight trialkyl amine having a stronger affinity to the carboxylic acid that is being recovered than the adsorber used in the first stage. In this way the aqueous solution of a water-soluble carboxylic acid ammonium salt is formed. This is then subjected to heat treatment, whereby the salt is decomposed to yield back the trialkyl amine or ammonia and free carboxylic acid. Applying this process to lactic acid involves the formation of salts of lactic acid with strong bases having a pKa value of about 9-11; i.e. the conjugate acid of the base has a pKa of 9-11. Thus, the decomposition of these salts into free lactic acid is energy intensive. Examples 12-14 of the ""456 patent mention the use of Alamine 336 (tricaprylylamine) for the extraction of, among others, lactic acid from an aqueous solution, but no yields are mentioned. Upon the extraction of even small quantities of lactic acid from a fermentation broth the pH of the broth rises rapidly to above 7. As shown in FIGS. 3 and 4 of the ""456 patent, the uptake of carboxylic acids from aqueous solutions drops rapidly with an increase of the pH. It is, therefore, inherent in these examples that the lactic acid uptake, if any, is negligible. It is further noted that upon heat treatment and concentration of an ammonium lactate, crystalline lactic acid does not precipitate and instead the viscosity of the solutions increases steadily as a result of self-association of the acid. It is thus evident that the process of U.S. Pat. No. 5,132,456 is unsuitable for the recovery of lactic acid from a fermentation broth.
U.S. Pat. Nos. 4,444,881 and 4,405,717 (Urbas) describe a process for the recovery of an organic acid from a diluted aqueous solution of its calcium salt by adding a water-soluble trialkyl amine carbonate to the solution to form on the one hand a water soluble trialkyl ammonium salt of the acid, which salt remains in solution, and on the other hand calcium carbonate which precipitates. After removal of the calcium carbonate the remaining mother liquor is heated for the separate recovery of the amine and the product acid. The decomposition of the trialkylammonium salts of this reference into free acids is energy intensive.
U.S. Pat. No. 4,282,323 (Yates) describes a process for obtaining lower carboxylic acids from a salt solution of such carboxylic acid as obtained from fermentation. The process appears to be applicable to a restricted number of lower aliphatic and aromatic monocarboxylic acids and is specifically described only in relation to acetic acid. In accordance with that process, the aqueous solution of a carboxylic acid salt is concentrated in the presence of a liquid polar organic solvent serving as extractant, with pressurized carbon dioxide, to convert at least part of the salt to the corresponding free acid which is taken up by the organic phase, from which it is subsequently recovered. It is inherent in the use of a polar organic extractant that the bulk of the carboxylic acid remains in the neutral to basic aqueous phase, and indeed the recovery rates reported in U.S. Pat. No. 4,282,323 are low, ranging between 4.8% and 18% of the acid initially present.
U.S. Pat. No. 4,275,234 (Baniel et al) is directed to a method of recovering various acids in their free form from aqueous solutions. Thus, the process of Baniel is not applicable to a lactate solution of the type commonly obtained from a fermentation process or from other sources. The essence of the Baniel et al. U.S. Pat. No. 4,275,234 is the discovery that efficient back-extraction can be achieved by performing the back-extraction at a temperature higher than that of the primary extraction.
R. Bar and J. L. Geiner, Biotechnology Progress, 3, 109 (1987) studied the feasibility of extracting lactic acid from aqueous solution by means of a long-chain trialkyl amine of low basicity, such as tridodecylamine, using various tridodecylamine solutions in n-dodecanol.
In accordance with the disclosure of U.S. Ser. No. 08/207,773, it was reported that it is possible to separate and recover lactic acid from a lactate solution at a pH in the range of 4 to 14 in a nearly quantitative fashion, with a desirable process. More specifically, the preferred lactic acid separation and recovery process reported includes an extraction hereinafter (sometimes referred to as the primary or forward extraction) in the presence of a water-immiscible, long-chain trialkyl amine and carbon dioxide. The lactate solution could be obtained from a fermentation broth or from hydrolyzed polylactide via polylactide recycling or recovery, among possible others.
In the parent disclosure, the invention is described as providing a process for the separation and/or recovery of lactic acid from a lactate solution formed by fermentation in the presence of a basic substance such as one selected from the group of alkali metal, alkaline earth metal or ammonium hydroxides, carbonates or bicarbonates. The process steps comprise obtaining a lactate feed solution from a fermentation broth or another source and combining such feed solution with an extractant. The particular preferred extractants disclosed are trialkyl amines, with the extraction being in the presence of carbon dioxide and with the trialkyl amine being water-immiscible and having a total of at least 18 carbon atoms. The term xe2x80x9ccombiningxe2x80x9d is explained in the parent as meaning a mixing or contacting of the lactate solution (aqueous phase) and the amine (organic phase) so that extraction can occur. It was recited that preferably the lactate feed solution is formed by filtering a fermentation broth to remove biomass and other solids and that the combining of the lactate solution and extractants preferably occurs in the presence of carbon dioxide at a partial pressure of at least about 50 psig.
The above extraction in accordance with the parent disclosure results in the formation of a lactic acid rich organic phase and an aqueous or aqueous-slurry phase. Each of these two phases, in accordance with preferred further aspects of the parent disclosure, is further processed: the processing being recovery of lactic acid from the organic phase and recovery of carbonate or bicarbonate from the aqueous phase. As explained in the parent, preferably the recovered carbonate or bicarbonate is recycled to the fermentor. The organic phase from which the lactic acid been recovered can be recycled for use in the primary extraction. If applied in the preferred manners described, this would result in a process in which the consumption of acids and bases is avoided and in which the generation of waste salts and other by-products is substantially reduced, if not eliminated.
In a preferred process of the parent disclosure, a countercurrent liquidxe2x80x94liquid extractor or extraction unit is used. During steady state operation, the lactate feed solution and extractant are loaded into the extractor and operated in the presence of pressurized carbon dioxide. The optimum operational pressure was described as not being critical, provided sufficient carbon dioxide is present for the primary extraction to occur. It was stated, however, that preferably the partial pressure of carbon dioxide is maintained at 50 psig or greater. It was described that upon leaving the extractor, the organic phase may be subjected to decompression. This would result in a release of the pressurized carbon dioxide which could, if desired, be recovered for reuse in the process.
The long-chain trialkyl amines described in the parent as useful and preferred are those in which the amines and the amine lactate salts are immiscible with water and have a total of at least 18 carbon atoms, and preferably have from 24 to 42 carbon atoms. Typical examples of such amines provided in the parent are trihexylamine, trioctylamine, triisooctylamine, tricaprylylamine and tridodecylamine. The parent disclosure states that the term xe2x80x9camine saltxe2x80x9d or xe2x80x9camine lactate saltxe2x80x9d refers to the species formed when lactic acid or lactate is extracted into the amine extractant phase, although the exact nature of this species is not known.
In the parent, the extraction process is described as being performed batchwise or continuously, but that dramatically improved separation and ultimate recovery can be achieved with a continuous process and in particular a countercurrent extraction process.
In the parent it is stated that solvents of the trialkyl amines may also be used, if desired, as part of the extractant. It is believed that these may be used for the purpose of diluting certain relatively viscous trialkyl amines, enhancing the extraction, and/or stabilizing and maintaining the organic phase in a single phase substantially immiscible with water. Any compatible organic solvent capable of dissolving the amine and the amine lactate salt would be suitable, provided it is also inert to chemical reaction both with the long-chain trialkyl amines utilized and to the amine lactate salt and lactic acid. In the parent, it was stated that the term xe2x80x9ccompatiblexe2x80x9d means miscible with, soluble in and chemically inert. The usefulness of solvents for these purposes is well known in the art. Specific examples, however, were described in the parent as liquid hydrocarbons such as kerosene or mineral oils, alkanols such as isopropanol, n-butanol and n-octanol and various ketones such as methyl-isobutyl ketone (MiBK) and nonanone, among others. In the parent it was stated that two or more different solvents may be used, for example a hydrocarbon and an alkanol.
According to the parent, the organic phase resulting from the primary or forward extraction is subjected to a separation process such as further extraction, vaporization or the like to recover the lactic acid. Also, it was stated that preferably the organic phase is subjected to back-extraction with water to recover the lactic acid in an aqueous phase. Where the initial extracting medium also contains an alkanol or ketone as a solvent, it was stated that the back-extraction may be preceded by removal of the solvent through azeotropic steam distillation or other techniques. It was also stated that the portion of the organic phase remaining after separation of the lactic acid and, where applicable, the separately recovered alkanol or ketone, can be recycled for use in the primary extraction. The aqueous lactic acid solution resulting from the back-extraction can be removed as product and can be concentrated, if desired.
In a preferred embodiment of the process described in the parent, carbonate or bicarbonate is present in the aqueous phase either in solution or as a solid suspension, predominantly in the form of an alkali metal, alkaline earth metal or ammonium carbonate or bicarbonate, depending on the cation present in the lactate solution. This aqueous phase is preferably a suspension of sodium bicarbonate crystals and is subjected to solid-liquid separation followed by conversion of the bicarbonate into sodium carbonate by heat treatment or other techniques known in the art. Carbon dioxide liberated during this conversion may be trapped and recycled for use in the primary extraction. The solid-liquid separation also yields an aqueous raffinate, substantially depleted of lactate, which is withdrawn and may be used as a constituent of animal feed.
In this section, a general discussion of the principles presented in the parent disclosure 08/207,773 and its parent 08/084,810, is provided. This section includes some additional comment on those principles, beyond the specific language of the ""773 and ""810 applications.
In the Background section of the parent disclosure, a technique for lactic acid recovery involving acidification of a lactate solution including calcium lactate, by sulfuric acid, was described. In general, it was stated that after the fermentation broth was acidified with sulfuric acid, using this prior art technique, and the mother liquor is concentrated, the lactic acid is extracted with an organic extractant. It was generally shown that next the organic phase (water-immiscible phase) can be back-extracted with water (or the acid is adsorbed on a suitable absorbent) for isolation and recovery of the lactic acid. This method is a general method involving the acidification of the broth with strong acids. A strong acid can substantially lower the pH of the lactate solution, causing formation of undissociated lactic acid (i.e.no lactate but essentially only lactic acid) and a salt of the strong acid. In the technique, an extractant is then used to extract the undissociated acid. A principal problem with this approach is the generation of large amounts of the salt of the strong acid used in the acidification process. In addition, the salt of the strong acid is typically too weak a base to be useful as a base in the fermentation broth to provide lactate salt.
Also in the Background of the parent, the Yates ""323 reference was discussed. It is noted that one of the carboxylic acids isolated during that process, is acetic acid. Acetic acid is a relatively weak organic acid. If a stronger organic acid were used in the process, it would typically be even more dissociated at a given or operating pH for the extraction, than was the acetic acid, unless a very basic operating pH were involved, i.e. one that ensured essentially all of the acid were dissociated. If the Yates"" process (including extraction with a liquid polar organic solvent as the extractant) were used, one would expect even lower recovery of a stronger organic acid than Yates achieved for acetic acid, under the conditions of the process, since the process concerns recovery of the acid, not the anion or base form. The recovery rates reported in Yates ""323 for acetic acid (4.8%-18%) were already relatively low. Thus, the Yates"" approach would not be expected to be very fruitful, if an acid substantially stronger than acetic acid, for example lactic acid, were involved.
The techniques developed and reported in the parent disclosures (i.e. Ser. Nos. 08/207,773 and 08/084,810) concern and take advantage of the fact that lactic acid is moderate acid, not an extremely strong one and not an extremely weak one. For purposes of the description, generally a xe2x80x9cstrong acidxe2x80x9d will be considered to be any acid with a pKa of 1.0 or less. A xe2x80x9cmoderate acidxe2x80x9d would be one having a pKa greater than 1.0 and no greater than about 4.0. xe2x80x9cWeak acidsxe2x80x9d would be those with a pKa greater than about 4.0. For purposes of classifying an acid according to this description, the PKa should generally be rounded to the nearest 0.1. In general, lactic acid, with a pKa of about 3.9, then, will be considered a moderate acid for purposes of this discussion. The pKa values in this paragraph refer to values at 25xc2x0 C.; i.e. room temperature.
In the parent disclosures, reference was made to conducting extractions with immiscible amines such as long chain trialkyl amines. These are typically moderate bases. In general a xe2x80x9cweakxe2x80x9d base will be considered herein to be a base with a pH of half neutralization of less than 2.5; a moderate base will be considered to be a base with a pH of half neutralization of between 2.5 and 7.0; and, a strong base will be considered to be a base with a pH of half neutralization of greater than 7.0. The term xe2x80x9cpH of half neutralizationxe2x80x9d is a measure of apparent basicity of a water-immiscible base, as defined in Grinstead, R. R. et al., J. Phys. Chem., Vol. 72 #5, p. 1630 (1968), incorporated herein by reference.
In general, the invention concerns a process for recovery of lactic acid. The process includes the steps of forming a multi-phase system including at least a first aqueous phase and a second organic or water-immiscible phase. The first aqueous phase is provided in a form having a pH of 4 to 14 and including lactate salt in solution. The organic or water-immiscible phase preferably includes an extractant capable of forming a water-immiscible lactate salt, with a lactate-containing component.
The process includes a step of extracting a first aqueous phase with a water-immiscible phase by forming a water-immiscible lactate salt with the extractant. The step of forming the lactate salt generally includes a step of acidifying at least one of the first aqueous phase and the water-immiscible phase without providing the first aqueous phase with a pH below 4. The process further includes a step of separating the resulting water-immiscible phase from a resulting aqueous phase, after the step of extracting; and, obtaining or generating lactic acid from the lactate salt of the extractant found in the water-immiscible phase. This later step is typically conducted through a form of back-extracting. The step of acidifying is preferably conducted by adding an acid to either or both of the first aqueous phase and the second water-immiscible phase. It may be conducted either before these two phases are brought together, or after.
Herein reference will, in some instances, be made to xe2x80x9clactate-containing componentxe2x80x9d in the aqueous phase. This term is intended to refer to both the anion form, lactate anion; and, the acid form, lactic acid, together. Preferred processes result in extraction of at least 90% of the lactate-containing component in the first aqueous phase, and generally and preferably are conducted until at least 95% of this material is extracted into the water-immiscible phase (as lactic acid). Indeed, preferably the process is conducted such that ultimately such amounts (i.e. at least 90%, and typically 95% or greater) are found in the final yield after the step of lactic acid recovery from the water-immiscible phase.
In general, the lactic acid separation and recovery process of the invention concerns a step of extracting the lactate-containing component with a water-immiscible weak or moderate base. However, it is preferred that the lactate-containing component removed be removed after the acidifying by addition of a moderate to weak acid. The particular preferred acid disclosed in the parent, is gaseous CO2, a weak acid which will generate carbonic acid in the solution. The carbonic acid, being a weak acid, will partly dissociate in the solution. Preferably the CO2 is added to the water-immiscible phase, before the multi-phase system is formed.
The use of the weak or moderate base for conduct of the extraction is important, since it facilitates later recovery of the lactic acid from the organic phase or layer. In particular, especially if a back-extraction with an aqueous phase or layer is used for final isolation of the lactic acid, the weak or moderate base will not hold the lactic acid sufficiently strongly to resist the partitioning of the lactic acid back into the aqueous phase. Preferably the back-extraction is not conducted with a weak base alone. Preferably it is conducted with a moderate base, or a mixture of weak base and moderate base.
The use of a relatively weak acid to acidify, prior to extraction or during extraction, is also important. The relatively weak acid will form a salt in the aqueous phase which is a relatively strong base that can be used directly, or after mild conversion (for example from a bicarbonate to a carbonate), as a base in the fermentation broth. The use of a moderate acid is less preferable because in general the salt formed will be too weak a base to be readily useful as a base in the fermentation broth.
In general, the weak or moderate base, used for the extraction, will be considered immiscible if, under the extraction conditions, it is sufficiently insoluble in the aqueous phase or layer such that its presence will be no greater than about 1000 ppm, and preferred ones will have a presence of no greater than about 200 ppm. Typically, bases will be chosen that have a solubility of 100 ppm or less. The weak or moderate base could be presented in the form of a solid, and thus be completely immiscible in water.
Also, preferably the weak or moderate base is also immiscible in the water used in the later back extraction, if one is conducted. A base will be considered immiscible in the aqueous phase of the back extraction if the above-stated ppm limits for water-immiscibility during the first extraction are not exceeded.
From the above it will be apparent that the process steps of preferred lactic acid recovery processes, described in the parent applications, concern the following steps:
(1) Obtaining a lactate feed solution from a fermentation broth or other source;
(2) Modifying the lactate feed solution, preferably with a source of moderate or weak acid while maintaining a pH of at least 4, and preferably within the range of 4-14; and
(3) Extracting the lactic acid with a water-immiscible weak or moderate base, or mixture.
In the preferred applications described, the water-immiscible weak or moderate base is a water-immiscible amine, typically an alkyl amine and preferably a water-immiscible tertiary amine. Preferably alkyl amines and most preferably water-immiscible trialkyl amines are used. As explained in the parent, the preferred water-immiscible amine will be a trialkyl amine containing at least a total of 18 carbon atoms.
In an alternate statement, typically when the extraction occurs, the aqueous phase is provided such that the lactate-containing component or species present, at any given time throughout the extraction, is at least 50% (molar equivalent) in the form of lactate, rather than the lactic acid. In general this is accomplished by appropriate control of the pH, and selection of the desirable acid for acidification of the system.
The preferred processes are conducted on lactate feed solutions that contain lactate values in a concentration of at least 3 mol. When the processes concern recovery from a fermentation broth, generally the fermentation process is conducted such that the feed solution contains at least 5%, and typically 10 to 30%, by weight, lactate. The broth may be concentrated, before extraction.
As also will be apparent from the parent disclosures, preferably the moderate or weak acid, used to acidify, is an acid which is either readily separated from the water-immiscible weak or moderate base, or which does not combine to any substantial extent with the water-immiscible weak or moderate base under the conditions of extraction. Carbon dioxide is an almost ideal acid for use in generation of the lactic acid, since its presence can so readily be controlled through control of its partial pressure, it can easily be removed from the solutions, it is relatively inexpensive, and it is such a weak acid that the salt which is generated is also very suitable for effective neutralization of fermentation broth. It is foreseen that in some instances the moderate or weak acid may comprise a salt of an acid having more than one proton; for example monosodium phosphate, provided the pKa for the remaining proton(s) is within the appropriate ranges.
In typical preferred applications, the acid which is used to acidify the multi-phase system is preferably a weaker acid (i.e. has a higher pKa) than lactic acid. A reason for this preference is that the corresponding salt of the added acid, which will form in the aqueous phase, will be a useful base in the fermentation broth. Also, when used with the preferred extractants, such as the water-immiscible amines, an advantage results because of the extractant""s preference for the stronger acid, i.e. lactic acid.
As indicated in the experiments, processes according to the invention are characterized by relatively high recoveries of the lactate values in the lactate containing feed solution (for example, the fermentation broth). Recoveries greater than 90% are readily achieved, and typically the recovery is at about 95% or greater.
With respect to the weak or moderate base, to be used in the extraction, as indicated in the parent application, it has been found that trialkyl amines having a total of at least 18 carbon atoms, and preferably from 24-42 carbon atoms, are most desired. Among the ones identified in the parent application, tridodecylamine presently appears preferred. However, in general it is believed that while tertiary amines are preferred, substituted tertiary amines and in some instances even primary or secondary amines may also be used, provided they are sufficiently water-immiscible and perform as weak or moderate bases. It is foreseen that, in many instances, primary amines may have sufficient water solubility to be undesirable with respect to possible contamination of the aqueous phase. It is also foreseen, that in some instances, primary or secondary amines may have too great a propensity to react with the lactic acid to form an amide, to be fully preferred. However, there is no theoretical reason why at least some secondary or primary amines could not be used, under certain circumstances.
Although the general principles have been described with respect to conduct of the extraction with either a weak base or a moderate base (or mixture of both), in general it is believed that if a weak base is used alone, the extraction results will not be as great as preferred because the weak base typically will prefer the undissociated acid, which was the weaker acid used for acidulation. The lactic acid, which is largely dissociated, is thus less effectively extracted by a weak base (when the weaker acid is used). Thus it is foreseen that if a weak base is to be used, it will generally be preferred to use it in a mixture with a moderate base as well. A weak base is typically used as a co-solvent (or solvent) in the water-immiscible phase, with a moderate base also present to facilitate extraction.
In the parent application, it was explained that while a variety of extraction processes may be used, it was foreseen that countercurrent extraction processes would be preferred. In addition to providing for countercurrent contacting of the two liquid phases, preferably the extraction unit used should provide for good removal and flow of the solids formed during precipitation of the salt of the moderate to weak acid. In the preferred processes described, this would be the bicarbonate salt of the cation in the fermentation broth. Typically the cation will be Na+. In the most preferred embodiments, wherein the lactate feed treated and extracted has a greater than 3 mol. lactate concentration and the extraction is conducted with a water-immiscible trialkyl amine, the aqueous phase is relatively small in volume and, may for a slurry with the solid precipitate.
In the parent applications, solvents and/or diluent for the trialkyl amines, i.e. solvents and/or diluents for the water-immiscible bases, were described. In general, the selection of solvent will be controlled by a variety of factors such as boiling point, water solubility, and ability to enhance the extraction efficiency of the water-immiscible moderate base. However, in general when the lactic acid recovery is from a lactate containing fermentation broth, it is desired that the extraction occur within as short a period of time of the generation of the fermentation broth as possible, for commercial efficiencies. Most fermentation broths are separated from the fermentation process at a temperature of about 50xc2x0 C., in typical commercial operations. Thus, unless time is used in substantial cooling, or cooling equipment is used, the extraction will typically occur of an aqueous solution having a temperature of at last 35xc2x0 C., usually about 40-50xc2x0 C.
If a back-extraction is used to recover the lactic acid from the organic phase, preferably the aqueous phase used during the back-extraction is at a higher temperature than the aqueous phase from which the lactic acid is separated in the first instance. This helps increase recovery efficiencies. Preferably the temperature of the back-extraction is as high as reasonably possible. Indeed, preferably it is conducted at a temperature of at least 100xc2x0 C., and typically and preferably at least 135xc2x0 C.
When a solvent and/or diluent is present in the organic phase, as will generally be preferred, the extraction is preferably conducted below the boiling point of the solvent and/or diluent. Thus, the temperature of the desired aqueous phase will, in some instances, dictate the preferred solvent used. It has been found that a mixture of paraffin (Isopar K, from Exxon) and octanol is a desirable solvent for the organic phase, and the use of octanol allows extraction temperatures up to about 140xc2x0-160xc2x0 C. The mixture would preferably comprise about 50% trialkyl amine, 30% n-octanol and 20% non-aromatic paraffin, by weight.
It is noted that in some instances the solvent (or co-solvent) in the water-immiscible phase will function as a weak base and thus be capable of some modest amount of extraction. This would be true, for example, when the solvent is an alcohol or a ketone. However, alcohols and ketones are generally such weak bases that their operation in extracting the lactic acid is more akin to a salvation process than a more tightly associated ion attraction with the lactic acid. Thus the presence of at least some moderate base, such as the amine bases, is generally preferred. The presence of a polar solvent such as an alcohol or ketone can enhance the ability of a moderate base, such as a water-immiscible amine, to extract an acid, such as lactic acid.
If the organic phase is back-extracted, for final recovery of the lactic acid, in some instances it may be preferable to conduct the back-extraction under carbon dioxide pressure, in order to facilitate the extraction. For example, if the primary extraction was conducted under carbon dioxide pressure, the conduct of the back-extraction under pressure will prevent CO2 release or the need to repressurize the escaping CO2.
In preferred applications, the step of acidifying includes acidifying the aqueous phase from a fermentation process, to form sodium bicarbonate in the aqueous phase; and, the process includes using the sodium bicarbonate to form sodium lactate in the first aqueous phase, in later processing. This latter step may include forming sodium carbonate from the sodium bicarbonate and then putting the sodium carbonate in a fermentation process.